Bump phpunit XSD version to 5.2

Some attributes being used in the phpunit configuration files, namely
failOnRisky and failOnWarning were introduced in phpunit 5.2.0. The
Composer configuration shows that tests should run with old versions of
phpunit, but phpunit only validates the configuration against the XSD
since phpunit 7.2.0.
These changes can be tested as follows:

wget http://schema.phpunit.de/5.2/phpunit.xsd
xargs xmllint --schema phpunit.xsd  1>/dev/null
find src -name phpunit.xml.dist| xargs xmllint --schema phpunit.xsd  1>/dev/null

See https://github.com/sebastianbergmann/phpunit/commit/7e06a82806be004cbfd30a02d92162e9ed4abc7c
See https://github.com/symfony/symfony/blob/46e3745a03e199e64cc0fcf3284a96b5a25dcee9/composer.json#L98